A thermosensitive recording medium designed to form a color image by melting and contacting a color former with a developer and making use of a color reaction by the two substances is known. For carrying out recording with such a thermosensitive recording medium, a recording system is generally employed in which the recording medium is run with its color developing layer in close attachment with a recording head (thermal head) having a heat generating element. In operation of such recording system, however, there are involved various problems such as wear of the head, adhesion of tailings to the head surface and sticking of the head with the color developing layer of the recording medium. Further, since the recording speed depends on the heat dissipation time of the thermal head, it is hardly possible to carry out high-speed printing and also there is a limitation to the resolution of the color image formed by diffusion of heat.
Recently, for real-time marking of letters and signs such as maker's name, product name, date of production, lot number, etc., on the surfaces of various commercial articles, for example, electronic parts such as IC's, resistors, condensers, inductors, etc., electrical parts such as relays, switches, connectors, printed circuit boards, etc., housings of the electrical devices, automobile parts, machine parts, cables, sheets, packaging sheets, cards, various containers of foods and medicines, caps of containers, etc., the laser marking system is popularly employed for its various advantages such as high speed printing, capability of fine marking, etc. Such laser marking system is essentially based on the principle that marking is made by breaking, that is, laser light is applied to the necessary part alone of the substrate surface to cause denaturing or removal of said part of the substrate, or laser light is applied to the coating film formed on the substrate surface to remove the coating film alone, so as to produce a contrast between the laser irradiated portion and the non-irradiated portion of the substrate.
When this laser marking method is applied to said color former and developer combination system, it may happen that break takes place even in the substrate to make it unable to perform desired marking. Also, when it is tried to lower the laser energy so as to prevent undesired break of the color developing part or the substrate, there arises the problem that the formed color image may fail to have the enough color density since the amount of energy applied per shot becomes less than 1.0 j/cm.sup.2 especially in case the color developing layer is very small in thickness, such as less than 5 .mu.m, or in case marking is carried out at a high speed of 30-40 shots/sec.
The present invention is aimed at providing a laser marking method using a color developing system comprising a color former and developer combination, which method is capable of non-break marking with clear and vivid color formation even when the color developing layer is a thin film of less than 5 .mu.m in thickness or when marking is carried out at high speed.